Growth hormone receptors in testis and liver during the spermatogenetic cycle in rainbow trout (Oncorhynchus mykiss)

Changes in growth hormone (GH) receptivity in the testis during the reproductive cycle were studied in rainbow trout (Oncorhynchus mykiss). This necessitated setting up a method to characterize GH receptors (GH-R) in crude testis homogenate. Binding of radiolabeled recombinant rainbow trout GH (125I-rtGH) to crude testis preparation was dependent on testicular tissue concentration. The sites were specific to GH, with no affinity for prolactins, gonadotropins, or insulin. Similar high affinities for 125I-rtGH were obtained with crude testis (4.5 x 10(9) + 0.25 M(-1)) and crude liver preparations (5.1 x 10(9) +/- 0.06 M(-1)) at all stages of spermatogenesis, as well as with testicular membrane preparations (2.4 x 10(9) M(-1)). GH-R concentration in testis was high during the immature stage (590 fmol/g testis); it then decreased regularly from the beginning (stage II, spermatogonia proliferation: 706 fmol/g testis) to the end of spermatogenesis (stage VII: 158 fmol/g testis). Conversely, the absolute testicular capacity increased dramatically between stages IV and VI (1.2-16.2 pmol/2 gonads), when testicular growth is extremely rapid. GH-R concentration in the liver was highest in early stages (6.5-6.7 pmol/g liver); this value then significantly decreased during midspermatogenesis (stage VI: 3.5 pmol/g liver) and returned to immature levels at the end of the cycle. No significant correlation was found between GH-R in testis, GH-R in liver, and plasma GH concentration. Preliminary experiments performed on isolated populations of testicular cells revealed that binding of 125I-rtGH was detectable in Sertoli cell-enriched fractions but not in germ cell populations. We conclude that GH-R are present in the testis during the entire reproductive cycle in rainbow trout, probably in somatic cells and in particular in Sertoli cells.